XX INTRODUCTION. 



III. ON THE METHODS OF KILLING ANIMALS. 



The method of killing is of considerable importance, and 

 careful attention should be paid to the directions given at the 

 commencement of each chapter. 



Two methods of Idlling animals are in general use, and the 

 one to be chosen depends upon whether the specimen is to be 

 (1) dissected and examined temporarily, -or (2) converted into 

 permanent microscopical preparations. 



For the first purpose chloroform is useful. Three or four 

 minutes' immersion is sufficient, after which the animal 

 should be washed in water. This answers well for the leech 

 and earthworm. Fo^ killing crayfish, boiling water may be 

 employed, and snails die well-expanded if drowned in warm 

 water. 



For the second purpose a reagent is employed which not 

 only kills the tissues, but penetrates and ' fixes ' them, so that 

 they are but slightly affected by subsequent treatment. The 

 choice of the reagent depends upon the size and texture of the 

 specimen, and the purpose for which it is intended (see 

 Appendix). Strong alcohol is the simplest and most gene- 

 rally applicable, but does not give the best results for histo- 

 logical work. For Protozoa osmic acid or the vapour of 

 glacial acetic acid is useful. For Hydra, Perenyi's fluid. 

 For larger animals a mixture of 90 parts concentrated corro- 

 sive sublimate solution and 10 parts glacial acetic acid is a 

 good reagent. The specimen must be subsequently washed 

 in alcohol. 



IV. ON DISSECTION. 



The object of dissection is to separate the several organs 

 from one another, so far as is necessary to define their boun- 

 daries and display their relations to one another. It consists 

 mainly in the removal of the connective tissue which binds 

 the parts together and obscures their outhnss. 



The necessary instruments are the following :■ 



1. Two or three scalpels, of different sizes. 



